Carbocations benzyhc

These theoretical considerations reveal that the empirical r values are intimately related to the theoretical indices of the structures of benzyhc carbocations derived from resonance theory. The coefficient r in the Y-T equation can thus be replaced as a first approximation by a set of theoretical quantities, e.g. increment of bond orders (PAr,a)H as in (35) or sum of charge populations in the aryl ring for the parent carbocations (X = H) as... 

Scheme 8.44. The reaction of formaldehyde with the phenoxide anion to produce ortho- and para-hydroxybenzyl alcohols. Acidification leads to benzyhc carbocations (an El elimination), which react with more formaldehyde, phenols, and substituted phenols to produce a formaldehyde-phenol resin-type polymer.

The relative ease of the reaction is attributed to the stabiUty of the carbocation intermediate (a benzyhc carbocation), just as we saw with Sjvjl reactions. 

Authors found that the vinylalumination reaction is fully diastereoconvergent as the reaction of both cis- and trans-stilbene oxides 90 led to the exclusive formation of the same anti-configured homoallyUc alcohol 39a, which was subsequently cyclized to fS,y-diphenyl-a-methylene-y-lactone trans-29aa under acidic conditions (Scheme 4.23). This experimental result was rationalized by the intermediacy of the corresponding benzyhc carbocation in the reaction. 

The first compound will generate a tertiary carbocation, while the second compound will generate a tertiary benzyhc carbocation that is resonance stabilized. The second compormd leads to a more stable carbocation, so the second compormd will lose its leaving group more rapidly than the first compormd. 

The carbonyl group is protonated, giving a resonance-stabilized intermediate that is then deprotonated to give an enol. Protonation of the enol results in a resonance-stabilized, benzyhc carbocation intermediate that is then deprotonated to give the product. 

The relative stabilities of radicals follow the same trend as for carhoca-tions. Like carbocations, radicals are electron deficient, and are stabilized by hyperconjugation. Therefore, the most substituted radical is most stable. For example, a 3° alkyl radical is more stable than a 2° alkyl radical, which in turn is more stable than a 1° alkyl radical. Allyl and benzyl radicals are more stable than alkyl radicals, because their unpaired electrons are delocalized. Electron delocalization increases the stability of a molecule. The more stable a radical, the faster it can be formed. Therefore, a hydrogen atom, bonded to either an allylic carbon or a benzylic carbon, is substituted more selectively in the halogenation reaction. The percentage substitution at allylic and benzyhc carbons is greater in the case of bromination than in the case of chlorination, because a bromine radical is more selective. 

Nakata et al., 1996, 1999). TTie agreement between the theoretical dihedral angles 0caic and the empirical 0expi of the twisted benzyhc cations confirms that the observed decrease in the r value should be ascribed to a loss of resonance interaction caused by deviation from coplanarity of the carbocation centre and the benzene rr-system. 

The Y-T equation (2) has been used extensively in studies of electrophilic substitution in the aromatic ring, and aliphatic nucleophilic substitution and related reactions forming a carbocation or a carbocationic (electron-deficient) centre at the conjugative position (mostly benzyhc position) in the side chain. 

The Hammett p value of -4.1 suggests a carbocation intermediate as does the regioselectivity of the reaction (MeOH attacks the benzyhc position) but the stereochemistry (the reaction occurs with inversion) and a modest negative entropy of activation (AS = -48 JmoH K ) suggest rather an Sn2 reaction with a loose transition state having substantial positive charge at the benzylic carbon. Neither piece of evidence alone would be enough to define the mechanism. 

Other electrophiles include acyHum ions produced from acid chlorides, carbocations from tertiary halides or secondary benzyhc halides, activated enones, and epoxides all in the presence of Lewis acid. In each case the new bond is highlighted in black. 

Section 11.14 Benzylic carbocations are intermediates in S l reactions of benzyhc halides and are stabilized by electron delocalization. 

The greater the number of alkyl substituents bonded to the positively charged carbon, the more stable the carbocation will be. The order of relative stability of carbocations is tertiary benzyhc > allylic secondary > primary vinyl> phenyl. The nature of electron release by alkyl groups is not very clear. It may be an inductive effect, a resonance effect (hyperconjugation), or a combination of the two. When we refer to the inductive effect of the alkyl groups, it should be clear that this might well include a contribution from hyperconjugation. 

If the preparation of 45 is typical, Friedel-Crafts alkylation appears to be just another example of electrophilic aromatic substitution where the reactive species is a carbocation. However, problems with a carbocation intermediate do not arise with other cations such as Br, CP, or NO2+. If a stable carbocation such as the benzyhc cation or a tertiary cation is the intermediate, the reaction is relatively straightforward. If a primary or secondary carbocation is formed, however, rearrangement to a more stable cation may occur before the reaction with benzene. 

The Ritter-type reaction of adamantane is accomplished using the NHPI/NO system. In this section, we show that NHPI combined with cerium ammonium nitrate (CAN) serves as an efficient system for the generation of both PINO from NHPI and carbocations from alkyl radicals. Thus, benzyhc compounds first undergo the amidation with alkyl nitrile under mild conditions to form amides in good yields. The reaction of ethylbenzene in the presence of CAN and NHPI in EtCN under argon at 80 °C for 6h produced N-(l-phenylethyl)propionamide in 84% yield at 61% conversion (Eq. (6.32)). The NHPI/CAN system can apply to the Ritter-type reaction of various alkylbenzenes and adamanatanes. 

Measurement of (R" /R ) can be accomphshed by cychc voltammetry for relatively stable species and by other methods for less stable cations. The values obtained for AGjj range from 83kcal/mol for the aromatic tropyhum ion to 130kcal/mol for destabilized benzyhc cations. For stable carbocations, the results obtained by this method correlate with cation stabihty as measured by pATr+. Some of these data are presented in Table 5.3. 

Primary and secondary aUyhc and benzyhc halides can react either by an 8 2 mechanism or by an SnI mechanism in ordinary nonacidic solvents. We would expect these hahdes to react by an Sn2 mechanism because they are structurally similar to primary and secondary alkyl hahdes. (Having only one or two groups attached to the carbon bearing the halogen does not prevent Sn2 attack.) But primary and secondary allylic and benzyhc hahdes can also react by an SnI mechanism because they can form relatively stable allylic carbocations and benzylic carbocations, and in this regard they differ from primary and secondary alkyl hahdes. ... 

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